This invention relates to a low-velocity electron excited phosphor, and more particularly to a phosphor which is excited by low-velocity electrons to emit a green luminous color and suitable for use for a fluorescent display device.
Japanese Patent Application Laid-Open Publication No. 149772/1976 discloses a low-velocity electron excited phosphor having a composition represented by the following general formula: EQU A(Zn.sub.1-x,Mg.sub.x)O Ga.sub.2 O.sub.3 :BMn
wherein 6.ltoreq.A.ltoreq.1.2, 0.ltoreq.B.ltoreq.5.times.10.sup.-2 and O.ltoreq.x.ltoreq.1.0.
The phosphor disclosed is prepared by adding Mn and Mg to a phosphor body or matrix of a blue luminous color represented by a formula ZnO.multidot.Ga.sub.2 O.sub.3.
FIG. 3 shows luminance life characteristics of phosphors including a phosphor of the present invention depending on a content of Al therein. In FIG. 3, a curve of X=0 indicates a relationship between relative luminance and operating time in a fluorescent display device including a ZnO.multidot.Ga.sub.2 O.sub.3 :Mn phosphor. The phosphor has a composition represented by the above-described formula wherein x=0.
As indicated by the curve of X=0 in FIG. 3, the phosphor is increased in luminance until about 3000 hours elapse after an initial stage of excitation of the phosphor and kept substantially unchanged thereafter, In general, a plurality of display segments arranged in a fluorescent display device are varied in frequency of excitations in use. Therefore, use of a phosphor exhibiting such luminance characteristics as described above for display segments of a fluorescent display device causes a difference in luminance to occur with time between the display segments subject to long-period excitation and those subject to short-period excitation in the same fluorescent display device, resulting in quality of a luminous display of the device being substantially deteriorated with time.